1. Field of the Invention
The present invention relates, in general, to a rotary compressor having a plurality of cylinders and, more particularly, to a rotary compressor which varies a compression capacity as desired, by selectively engaging one or a plurality of roller pistons according to a direction of rotation of a rotating shaft which drives the rotating pistons.
2. Description of the Related Art
As is well known to those skilled in the art, compressors are widely used in a variety of refrigeration systems, such as refrigerators and air conditioners. In such refrigeration systems, the compressor compresses refrigerant to highly pressurize the refrigerant prior to discharging the high-temperature and high-pressure refrigerant to a condenser. The compressors are typically classified into linear compressors, reciprocating compressors and rotary compressors. The present invention relates to a rotary compressor compressing a refrigerant by a roller piston which is arranged in a cylinder and is eccentrically rotated. More particularly, the present invention relates to a rotary compressor which is provided with a plurality of cylinders and which varies a capacity of the rotary compressor.
A conventional rotary compressor of a double cylinder structure will be now be described. Referring now to FIG. 1, a conventional rotary compressor includes a hermetic casing 100, with a drive unit 10 and a compressing unit 11 installed in the casing 100. A rotating shaft 101 is arranged at a center of the drive unit 10, and is provided with first and second eccentric parts 101a and 101b. A cylindrical rotor 102 surrounds the rotating shaft 101 and is rotated by an electromagnetic force. A cylindrical stator 103 surrounds the rotor 102 at a position which is spaced apart from the rotor 102 by a predetermined interval, and is fixedly mounted to the casing 100, with a coil wound around the stator 103. Further, a weight balancer 104 is provided at the bottom of the rotor 102 so as to reduce vibration and noise of the compressor due to an imbalance of the center of the rotation of the eccentric parts 101a and 101b. The compressing unit 11 includes the first and second eccentric parts 101a and 101b of the rotating shaft 101, and first and second cylinders 106a and 106b in which first and second roller pistons 105a and 105b are arranged. The upper surface of the first cylinder 106a is hermetically closed by an upper flange 107 which supports the rotating shaft 101, while the lower surface of the first cylinder 106a is closed by a middle plate 108. In this case, the middle plate 108 is positioned between the first and second cylinders 106a and 106b to hermetically separate a compression chamber 201a of the first cylinder 106a from a compression chamber 201b of the second cylinder 106b. Similarly, the lower surface of the second cylinder 106b is hermetically closed by a lower flange 109 which supports the rotating shaft 101, while the upper surface of the second cylinder 106b is closed by the middle plate 108. In such a rotary compressor having a double cylinder structure, after a refrigerant is compressed in the compressing unit 11 by a rotating force of the drive unit 10, the compressed refrigerant is discharged to the outside of the cylinder 106. Next, the refrigerant is discharged to the outside of the compressor through a refrigerant outlet pipe 110, and then flows into a condenser (not shown). In FIG. 1, the reference numeral 111 denotes an oil container for containing oil therein. Several components of the compressor are smoothly operated due to the lubricating effect of the oil.
The operation of the rotary compressor having a double cylinder structure will be described with reference to FIG. 2, which is a sectional view of one of the first and second cylinders 106a or 106b included in the compressor.
When the rotating shaft 101 is rotated in a direction as shown by an arrow in FIG. 2, the roller piston 105 is eccentrically rotated while being in contact with an inner circumferential surface of the cylinder 106, by the rotation of the eccentric part 101a or 101b, provided on the rotating shaft 101. During the rotation, a space distribution within a compression chamber 201, which comprises an intake part 21a and a discharge part 21b, is varied. That is, the intake part 21a becomes large in volume while becoming low in pressure, so refrigerant of an accumulator 112 is sucked into the intake part 21a through an intake hole 202. As the volume of the discharge part 21b becomes small due to the rotation of the roller piston 105, the refrigerant in the discharge part 21b is highly pressurized. Thus, the highly pressurized refrigerant is discharged to the outside of the cylinder 106 through an outlet hole 203. Thereafter, the refrigerant is discharged to the outside of the compressor through the refrigerant outlet pipe 110. The intake part 21a and the discharge part 21b are hermetically separated from each other by a vane 204 which is biased by a spring 204a, thus preventing the refrigerant from flowing between the intake part 21a and the discharge part 21b. 
However, the conventional rotary compressor having the double cylinder structure has a problem that excessive vacuum may be generated in the discharge part 21b of the cylinder 106 when the rotating shaft 101 is rotated in a reverse direction, so the compressor may be broken. Thus, the conventional rotary compressor uses a motor which rotates the rotating shaft 101 in a single direction. Therefore, the first and second cylinders 106a and 106b and other associated components are constructed such that the refrigerant is compressed during a single directional rotation of the rotating shaft 101, so only a compressing action is ever performed in the first and second cylinders 106a and 106b. Thus, an expensive inverter circuit is required to vary a compression capacity of such a compressor. Moreover, a control board is additionally required to control the inverter circuit, thus undesirably increasing a production cost of the compressor and increasing power consumption when the compressor is operated.
A reciprocating compressor having a construction for varying a compression capacity is disclosed in U.S. Pat. No. 6,132,177. However, such a construction is applied to only a reciprocating compressor. Substantially, there has not been developed a rotary compressor having a construction for varying a compression capacity as desired. In addition, the design of a rotary compressor having a construction which varies a compression capacity has been recognized as being very difficult.